In U.S. Pat. Nos. 4,156,838 and 4,095,163 there are disclosed power line transient suppression circuits including shunt branches with bi-directional voltage breakdown devices in series with a capacitor. Each suppression circuit also includes a series branch containing an inductor. At normal power line frequencies, such as 50 to 60 Hertz, the impedances of the inductor and capacitor are such that there is no significant loading of the lower impedance line and a source connected to it by the transient suppression circuit because a low-pass filter formed by the inductor and capacitor has a cutoff frequency greater than the frequency of the power source. The breakdown device conducts in response to the voltage developed across it exceeding a predetermined breakdown potential, typically 1.5 volts for a 115 volt AC power line. The bi-directional breakdown device becomes conducting shortly after a zero crossover of the AC power source in both directions, during each half-cycle of the power source.
Typically, transient noise pulses have components with frequencies considerably higher than the frequency of the power source, generally in excess of 5 kiloHertz, and possibly as high as several megaHertz. The high frequencies associated with the noise pulses have a tendency to cause ringing or oscillation of the series inductance and shunt capacitor, whereby high frequency, high amplitude current pulses have a tendency to be coupled to the load with possible deleterious effects.
It has now been discovered that relatively high current surges have a tendency to be developed in the shunt branch in response to the transients. The surges are frequently of sufficiently high amplitude to cause breakdown of the capacitor dielectric, thereby tending to destroy the capacitor. Destruction of the capacitor cannot be tolerated because it renders the filter circuit inoperative. The problem is most pronounced in high quality capacitors, having a capacity of 1 microfarad or more. High quality capacitors with such large capacitance values typically have impedances of less than 1 ohm at frequency ranges of 1 megaHertz and higher, frequencies that are often derived in the shunt branch in response to transient pulses on the line. The low impedance of the high quality, relatively large capacitors tends to increase the amplitude of the destructive voltages developed across the capacitors. Thus, in the prior art an anamolous situation has frequently developed in that high quality, expensive capacitors are desirously employed to minimize low frequency, power line losses, but these same capacitors are easily destroyed in response to noise pulses being coupled to the power line.
It is, accordingly, an object of the present invention to provide a new and improved power line transient suppression.
Another object of the invention is to provide a new and improved power line transient suppression circuit having a shunt branch including a capacitor and a breakdown device in series, wherein the tendency for destructive currents to be developed in the capacitor in response to transient pulses on the power line is obviated.
A further object of the invention is to provide a power line transient suppression circuit including a low-pass filter having an inductive series branch and a shunt branch including a capacitor and a breakdown device, wherein the inductor and capacitor have a tendency to cause high amplitude resonant destructive ringing currents to be established in the load in response to transients and a dead band of the breakdown device causes current of the branch to be interrupted, wherein destructive surges having a tendency to be coupled to the capacitor in response to the transients are effectively eliminated.